Materials dependence of the spin-momentum transfer efficiency and critical current in ferromagnetic metal/Cu multilayers

We have quantitatively compared spin-momentum transfer effects in FM/Cu multilayers, with $\mathrm{FM}=\mathrm{Co},$ ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10},$ ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20},$ ${\mathrm{Ni}}_{40}{\mathrm{Fe}}_{10}{\mathrm{Cu}}_{50},$ and Fe. All of the measured multilayers exhibited an abrupt step up in resistance at a critical current $I_c,$ the value of which was a linear function of applied magnetic field $H_{\mathrm{app}}.$ The spin-transfer efficiencies determined from these data were in substantial agreement with their predicted values over the range of materials studied. $I_c{\mathrm{(H}}_{\mathrm{app}}\mathrm{),}$ when evaluated at zero net field, was found to increase with the exchange energy density of the ferromagnet, demonstrating the tunability of the critical current by materials selection.